Drug delivery systems such as diblock and triblock copolymers have been used to deliver a variety of drugs and are generally formulated to deliver specific drugs whether they are hydrophobic drugs or hydrophilic drugs. Depending on the drug solubility these drug formulations differ in polymer concentrations, types of polymers utilized, molecular weights of the polymers and solvents used in the formulations.
Also the type of environment in which the drug is delivered is an important consideration in formulating a drug delivery system. Thus, there exist drug delivery compositions that are prepared using temperature sensitive polymers, phase sensitive polymers, pH sensitive polymers and photosensitive polymers. See, for example, K. Al-Tahami and J. Singh “Smart Polymer Based Delivery Systems for Peptide and Proteins,” Recent Patents on Drug Delivery & Formulation, 1: pages: 65-71 Bentham Science Publishers, LTD. 2007.
U.S. Pat. No. 6,592,899 describes a PLA/PLGA oligomer combined with a block copolymer for enhancing the solubility of a hydrophobic drug into a hydrophilic environment. More specifically this polymer composition has a polyester oligomer having a molecular weight of between 400 and 10,000 daltons and a biodegradable AB-type, ABA-type or BAB type block copolymer. The hydrophobic A part is a polyester, while the hydrophilic B part is a polyethylene glycol having a molecular weight of between 2,400 and 4,999 daltons. This polymeric composition is soluble in an aqueous environment.
U.S. Pat. No. 6,541,033 describes a sustained release pharmaceutical composition based on thermosensitive, biodegradable hydrogels, consisting of a block copolymer of PLA or PLGA and PEG, for the sustained delivery of biologically active agents, such as leptin. The sustained release is for a period of a week or more and preferably up to one month.
Hydrogels containing triblock copolymers are described in U.S. Pat. No. 6,350,812. These hydrogels retain water weight at least equal to the water weight of the copolymer and are soft hydrogels.
None of the patents nor the literature cited above describes drug delivery compositions that are injectable, in situ forming and are biodegradable and turn into solid implants when injected into the body. The biodegradable drug compositions of the present invention comprise triblock copolymers and diblock copolymers formulated in such a manner that the diblock copolymer serves as a reservoir while the triblock copolymer acts as a frame in the formulations and increases the lifespan of the diblock copolymer.
Furthermore, the biodegradable drug delivery compositions of the present invention can be long acting formulations, which reduce the initial burst release of the drug and modulate the release rate of the drug over time. This phenomenon is illustrated in the flattening of the drug release curves.